Frequency reducing arrangement



- 1,633 481 June 21, 1927. FALLOU FREQUENCY nnnucme ARRANGEMENT Filed Nov. 1926 Fly. I

mveni'ak Patented June 21, 1927.

UNITED STATES PATENT OFFICE.

JEAN GUSTA'V'E FALLOU, OF PARIS. FRANCE, ASSIGNOR TO IJ'UNION GENERALE DELECTBICITE, 01E PARIS, FRANCE.

FREQUENCY REDUCING ARRANGEMENT.

Application filed November 8, 1926, Serial No. 147,143, and in France January 19, 1926.

My invention has for its object an arrangement allowing to reduce the frequency of an alternating current to a secondary frequency which is a given fraction of the pri- 5 mary frequency. This is of great interest for radiocomniunication when it is desired to transmit or to receive waves at a frequency lower than that of the primary oscillations.

Generally speaking it can be used in any electric circuit comprising a generator, a motor and a measuring instrument each time it is necessary to provide this circuit with a reduced frequency current.

Up till now this frequency reduction has been obtained only through rotating machines or by means of, vacuum tubes. Now rota-ting machines can only be used for low frequencies and vacuum tubes cannot be used otherwise than for very small powers.

According to my invention I reduce the frequency by making the primary oscillation act on an oscillating circuit the frequency of which is multiple and comprises a fundamental frequency and several harmonics one of which has the same frequency as that of the primary oscillation.

Under such conditions any phenomenon making this circuit oscillate freely will be sufficient .to allow the primary oscillation to keep up the oscillations of the said circuit, these oscillations comprising the funda mental frequency oscillations and their harmonies. Electric filters of any known type allow an eas elimination of all the nude sired oscillations, only the desired harmonic assing through the filter. Any oscillation 5 :fiirmgehe frequency of the fundamental oscillation of the oscillating circuit or of one 4 of its harmonics can thus be obtained.

For instance a circuit of the kind described can be provided as explained hereinafter by means of a circuit comprising an inducion coil with a magnetic core. The

free oscillations may be excited with their harmonics in such a circuit by a sudden modification to the electric state of the said circuit, for instance by opening a switch inserted so as to short-circuit the induction coil.

The following explanations allow the means whereby this can be obtained to be readily understood with reference to appended drawings.

Fig. 1 is an explanatory diagram. 65

Fig. 2 is a diagram given by way of example of a form of execution of a frequency demultip'lying arrangement.

Fig. 1 shows an oscillating circuit comprising inseries a supply of alternating current of a frequency N, a condenser 2 having a capacity C, a switch 3 and an induction coil 4 of an inductance L and having no magnetiocore.

The supply 1 being active and the switch 3 being first open there will be produced on closing of the switch a varying voltage between the terminals of the condenser the value of which is the sum of-- 1/ a nonpermanent amount due to a damped free oscillation of the circuit the frequency of which is approximately 1 21r /LC This amount becomes equal to zero after a very short time, which varies. according to the damping factor i. e. double the ratio between the resistance and the inductance.

2/ a permanent amount due to oscillations having the same frequency N as the supply.

If it is desired for the supply 1 to keep up continuously the free oscillations of the circuit which can be done only at the frequency m of these oscillations, it is necessary that the frequency N of the supply or of its harmonics should be equal to the frequency m of the free oscillations of the circuit. Otherwise stated, one should have N:m or KN=m (KN being the frequency of the Nth harmonic when it exists).

This proves that the oscillations of a circuit having-a frequency m cannot be kept up by an alternator for instance without a special arrangement unless the frequency N of the said alternator is a submultiple of or is equal to the frequency of the waves it is desired to produce. Therefore a given circuit can only serve for transmitting the frequency of the source or a multiple thereof.

If in the diagram shown on Fig. 1 the induction coil 4 without a magnetic core is replaced by an induction coil provided with a l 21r /ij and several harmonics of this fundamental wave having a frequency equal to multlples of m.

It is easy to understand that if the frequency N of the supply 1 is equal to that of one of the harmonics of the fundamental wave which wave and harmonics are produced by the closing of the switch, the whole system comprising this wave and its harmonies will be kept up. A predeternnned wave can be therefore selected, the other waves being eliminated by suitable known electric filters and the remaining wave will have the desired frequency lower than that of the supply.

It is not necessary for the keeping up of the oscillations that the current from the supply and the harmonic having the same frequency should be in phase. However if this were desired, the putting into phase could be easily provided by closing the switch at the instant the supply current passes through zero.

Experience has shown the above theory to be correct and I have been able to keep up in a small-sized circuit fed by a 50 period alternator oscillations having a frequency of 50/9 of 50/4 and of 50/3; this latter frequency was kept up with a great steadiness and an output of about 17 watts. 7 By way of example I have described hereinbelow a diagram of a form of execution of my invention, this diagram being shown on Fig. 2. An alternating supply 1 of frequency N is coupled by a transformer 5 with an oscillating circuit comprising in series a condenser 2 having a capacity C .and a coil L provided with a magnetic core .and having an inductance L in the straight part of its characteristic curve (i. e. when its core is not saturated).

This coil is short circuited by the switch 3.

The apparent inductance due to the supply will be supposed equal to L.

The frequency of the free fundamental oscillations of this circuit, the iron not being saturated, is:

l 21r /(IFL6C and that of its harmonics is:

in 21r /(L+L')O K being an integer.

If the frequency N of the alternating supply A is equal to that of one of these harmonics that is if or iii (L L)O Mag) 1 there is resonance and the fundamental oscillation of frequency will be kept up in the circuit together with its harmonics.

To set the circuit oscillating freely it is sufficient to suddenly modify the electrical constitution of the circuit; thus the coil 4. can be short-circuited by means of the switch 3 which is thenafter reopened. The current is cut off by the switch at its passage through zero that is when the voltage of the condenser is at its lowest.

To use the desired alternating current having a frequency the primary of a transformer 6 having an inductance L is preferably connected with the terminals of the condenser 2'.

The condition for resonance will then be:

the term due to the inductance being modified by the action of the inductance L.

The current produced at the terminals of the secondary of the transformer 6/by the fundamental wave for instance will have the desired demultiplied frequency The its harmonics and means for selecting the desired oscillations.

2. Frequency reducing arrangement comprising a circuit adapted to oscillate freely ata fundamental frequency and at the harmonies thereof, a supply of alternating current the frequency of which is equal to that of one of these harmonics and coupled with the said circuit, means for starting the free oscillations of the circuit, means for selecting the desired oscillations, and a utilization circuit coupled with the oscillating circuit.

3. Frequency reducing arrangement comprising a circuit adapted to oscillate freely at a fundamental frequencyoand at the harmonies thereof, aninduction coil inserted in the said circuit and provided with a magnetic core adapted to be saturated by a current weaker than that due to the free oscillations of the circuit, a supply of alternating current the frequency of which is equal to that of one of these harmonics and coupled -with the said circuit means for starting the free oscillations of the circuit, means for selecting the desired oscillations, and a utilization circuit coupled with the oscillating circuit.

4, Frequency reducing arrangement comprising a circuit adapted to oscillate freely at a fundamental frequency and at the barmonies thereof, an in uction coil inserted in 5. Frequency reducing arrangement com-- prising a circuit adapted to oscillate freely at a fundamental frequency and at the harmonies thereof, an induction coil inserted in the said circuit and provided with a magnetic core adapted to be saturated by a current weaker than that due to the free oscillations of the circuit, a switch adapted to short circuit the said coil, a condenser in said circuit, a second coil inserted in parallel with-the said condenser, a supply of alternating current the frequency of which is equal to that of one of these harmonics and coupled with the said-circuit, means for selecting the desired oscillations, and a utilization circuit coupled with the oscillating circuit through the coil inserted in parallel with the condenser.

In testimony whereof I have aflixed my signature.

JEAN GUSTAVE FALLOU. 

